[chapium]

Article mentions coal based heating being a big culprit, which is most likely used residentially in homes and small businesses.

[Kayou]

I thought it was a central source of coal heating, but if it's in every home, it's probably too difficult to filter then.

[seanmcdirmid]

In city heating is typically central, in then countryside it is by home. When I visited Tianjin in 1999, however, the apartment I was staying at had a coal furnace.